A002644 Numbers k such that (k^2 + k + 1)/21 is prime.

16, 25, 37, 46, 58, 88, 109, 130, 142, 151, 184, 193, 205, 247, 268, 298, 310, 319, 331, 340, 382, 394, 403, 415, 424, 457, 478, 487, 541, 550, 604, 613, 688, 697, 709, 730, 739, 751, 760, 793, 844, 865, 886, 907, 970, 1012, 1045, 1054, 1066, 1117, 1138

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• easy

References

• A. J. C. Cunningham, Binomial Factorisations, Vols. 1-9, Hodgson, London, 1923-1929; see Vol. 1, pp. 245-259.
• N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
• N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000
• A. J. C. Cunningham, Binomial Factorisations, Vols. 1-9, Hodgson, London, 1923-1929. [Annotated scans of a few pages from Volumes 1 and 2]

Programs

Mathematica

Select[Range[1200], PrimeQ[(#^2 + # + 1)/21] &] (* Vincenzo Librandi, Sep 25 2012 *)

PARI

is(n)=isprime((n^2+n+1)/21) \\ Charles R Greathouse IV, Jun 06 2017

Extensions

More terms from Larry Reeves (larryr(AT)acm.org), Sep 21 2000